When I was younger I was more into the Pacific - I think I loved the naval aviation story. But in much more recent time, I have really been into Europe and the Battle of Britain in particular. I think it's pretty incredible how it all unfolded: a force of roughly 800 interceptors fought off an attacking force of 3,000 due to a brilliant strategy with a new technology that wasn't fully appreciated by the enemy. Then the whole thing changes after a navigation error brings civilians into the horror of war but saves the defensive force - and the British Isles effectively defeat any prospect of a Nazi invasion.
I have been so into it at times that I would read some of the day-by-day diaries that are online. They were teetering on the knife's edge for a while there. Must have been terrifying to live in England at the time.